🥀Intro to Botany Unit 2 – Plant physiology

Key Concepts and Terminology

• Understand the fundamental differences between plant and animal cells, including the presence of cell walls, chloroplasts, and vacuoles in plant cells
• Define and explain the roles of key plant cell organelles such as the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus
• Differentiate between primary and secondary growth in plants and their respective tissues (apical meristems, lateral meristems)
• Explain the concept of turgor pressure and its importance in plant cell structure and function
• Describe the process of osmosis and its role in plant water relations
• Understand the concept of transpiration and its significance in plant water transport and temperature regulation
• Define photosynthesis and its two main stages: light-dependent reactions and the Calvin cycle
• Explain the role of stomata in gas exchange and water regulation in plants

Plant Cell Structure and Function

• Plant cells are eukaryotic cells characterized by the presence of a cell wall, a large central vacuole, and plastids such as chloroplasts
  • The cell wall provides structural support and protection, and is composed primarily of cellulose and other polysaccharides
  • The large central vacuole stores water, nutrients, and waste products, and helps maintain turgor pressure
• Chloroplasts are the site of photosynthesis, where light energy is converted into chemical energy in the form of glucose
• Mitochondria are responsible for cellular respiration, generating ATP for energy-requiring processes in the cell
• The endoplasmic reticulum and Golgi apparatus play crucial roles in protein synthesis, modification, and transport
• Plasmodesmata are channels that connect adjacent plant cells, allowing for communication and transport of materials between cells
• Plant cells can differentiate into various specialized cell types (parenchyma, collenchyma, sclerenchyma) to perform specific functions within the plant
• Cell division in plants occurs in specific regions called meristems, which are responsible for primary and secondary growth

Photosynthesis and Energy Production

• Photosynthesis is the process by which plants convert light energy into chemical energy in the form of glucose, using carbon dioxide and water as raw materials
• The two main stages of photosynthesis are the light-dependent reactions and the Calvin cycle (light-independent reactions)
  • Light-dependent reactions occur in the thylakoid membranes of chloroplasts and involve the absorption of light energy by chlorophyll pigments
  • The Calvin cycle takes place in the stroma of chloroplasts and uses the energy-rich compounds (ATP and NADPH) produced in the light-dependent reactions to fix carbon dioxide into glucose
• Chlorophyll is the primary pigment responsible for absorbing light energy in photosynthesis, with chlorophyll a being the most important for driving the light-dependent reactions
• Photosystems I and II are protein complexes involved in the light-dependent reactions, each containing a reaction center chlorophyll molecule (P700 and P680, respectively)
• The electron transport chain and chemiosmosis are crucial processes in the light-dependent reactions that generate ATP and NADPH for use in the Calvin cycle
• C3, C4, and CAM plants have different adaptations for carbon fixation, with C4 and CAM plants being more efficient in hot, dry environments
• Factors affecting photosynthesis include light intensity, carbon dioxide concentration, temperature, and water availability

Water Relations and Transpiration

• Water is essential for plant growth and development, serving as a solvent, transport medium, and reactant in various cellular processes
• Water potential is a measure of the potential energy of water in a system, with pure water having a water potential of zero
  • Components of water potential include osmotic potential (solute concentration) and pressure potential (turgor pressure)
  • Water always moves from areas of high water potential to areas of low water potential
• Osmosis is the movement of water across a semipermeable membrane from a region of high water potential to a region of low water potential
• Turgor pressure is the pressure exerted by the cell contents against the cell wall, and is essential for maintaining cell shape and rigidity
• Transpiration is the loss of water vapor from plant leaves through stomata, and is driven by the water potential gradient between the leaf and the atmosphere
  • Factors affecting transpiration include light intensity, temperature, humidity, and wind speed
• The cohesion-tension theory explains the transport of water from roots to leaves through the xylem, driven by the evaporation of water from leaf surfaces
• Plants have various adaptations to conserve water and maintain water balance, such as leaf modifications (cuticle, trichomes), stomatal regulation, and root system architecture

Nutrient Uptake and Transport

• Essential plant nutrients include macronutrients (nitrogen, phosphorus, potassium) and micronutrients (iron, zinc, boron), which are required for proper growth and development
• Nutrient uptake occurs primarily through the roots, with nutrients moving from the soil solution into root cells via active transport, passive transport, or facilitated diffusion
  • Active transport requires energy (ATP) and is used to move nutrients against their concentration gradient
  • Passive transport occurs down the concentration gradient and does not require energy input
• The casparian strip in the endodermis of roots acts as a barrier to the apoplastic pathway, ensuring selective nutrient uptake and transport
• Xylem is responsible for the transport of water and dissolved mineral nutrients from the roots to the shoots
• Phloem is responsible for the transport of sugars and other organic compounds from source (leaves) to sink (roots, fruits) tissues
• Translocation is the process by which sugars and other organic compounds are transported through the phloem, driven by the pressure flow hypothesis
• Nutrient deficiencies can lead to visible symptoms (chlorosis, necrosis) and impaired plant growth and development
• Fertilizers are commonly used to supplement soil nutrients and promote plant growth, but excessive use can lead to environmental problems (eutrophication)

Plant Growth and Development

• Plant growth and development are regulated by a combination of genetic, environmental, and hormonal factors
• Meristems are regions of actively dividing cells that give rise to new tissues and organs
  • Apical meristems (shoot and root tips) are responsible for primary growth, increasing plant height and root length
  • Lateral meristems (vascular cambium, cork cambium) are responsible for secondary growth, increasing stem and root diameter
• Plant hormones (auxins, cytokinins, gibberellins, ethylene, abscisic acid) play crucial roles in regulating various aspects of plant growth and development
  • Auxins promote cell elongation, apical dominance, and root formation
  • Cytokinins stimulate cell division, delay senescence, and promote shoot formation
  • Gibberellins promote stem elongation, seed germination, and fruit development
• Photoperiodism is the response of plants to the relative lengths of day and night, which influences flowering and other developmental processes
  • Short-day plants (poinsettia) flower when the night length exceeds a critical threshold
  • Long-day plants (spinach) flower when the night length falls below a critical threshold
• Vernalization is the requirement of a period of cold exposure to induce flowering in certain plants (winter wheat)
• Senescence is the programmed aging and death of plant tissues, which is influenced by hormones, nutrient availability, and environmental factors

Plant Responses to Environmental Stimuli

• Plants are sessile organisms that must adapt to their environment through various physiological and morphological responses
• Phototropism is the growth response of plants to light, with shoots exhibiting positive phototropism (growth towards light) and roots exhibiting negative phototropism (growth away from light)
• Gravitropism is the growth response of plants to gravity, with roots exhibiting positive gravitropism (growth towards gravity) and shoots exhibiting negative gravitropism (growth away from gravity)
• Thigmotropism is the growth response of plants to touch or contact, as seen in the twining of vines and the closure of Venus flytrap leaves
• Nastic movements are non-directional responses to stimuli, such as the opening and closing of flowers in response to light and temperature changes
• Stress responses in plants include adaptations to drought (reduced stomatal conductance, increased root growth), salt stress (ion exclusion, osmotic adjustment), and temperature extremes (heat shock proteins, cold acclimation)
• Plant defense mechanisms against herbivores and pathogens include physical barriers (thorns, trichomes), chemical defenses (secondary metabolites), and induced responses (systemic acquired resistance)

Practical Applications and Research

• Understanding plant physiology has numerous practical applications in agriculture, horticulture, and biotechnology
• Crop improvement through breeding and genetic engineering can increase yield, nutritional quality, and resistance to biotic and abiotic stresses
  • Examples include the development of high-yielding wheat varieties during the Green Revolution and the creation of genetically modified crops (Bt cotton, Golden Rice)
• Precision agriculture uses remote sensing, GPS, and other technologies to optimize crop management practices (irrigation, fertilization, pest control) based on site-specific conditions
• Controlled environment agriculture (greenhouses, vertical farms) allows for year-round production and precise control over growing conditions, increasing efficiency and sustainability
• Bioremediation uses plants to remove pollutants from soil and water, such as the use of poplar trees to absorb heavy metals and organic contaminants
• Plant-based pharmaceuticals and industrial products are being developed, such as the production of vaccines and biodegradable plastics in transgenic plants
• Research in plant physiology continues to advance our understanding of plant growth, development, and responses to environmental challenges, with implications for improving crop productivity and resilience in the face of climate change